first - see that the first fall of rider is under the section "faults at obstacle" meaning fall occurs at an obstacle.
Second there is a zone around the fence - I believe more detail in the rule book.
Either way the rule clearly says rider can dismount voluntarily or involuntarily on course without penalty as long as it is not at an obstacle. See bold.
4. PACE AND DISMOUNTING. Between the start and finish, competitors are free to
Choose their own pace. They must be mounted to pass all red and white boundary flags. Dismounting as a result of attempting an obstacle, whether voluntary or not, is penalized in accordance with EV141.1.b. Elsewhere on the course, competitors may dismount without incurring a fault, other than possible penalties for exceeding the optimum time.
Pg.44 (at obstacle)
EV141 cross-country scoring.
1. FAULTS AT OBSTACLES:
A. Disobediences -
(1) First refusal, run-out or circle 20 penalties
(2) Second refusal, run-out or circle at the same obstacle 40 penalties
(3) Third refusal, run-out or circle at the same obstacle Elimination
(4) Fourth penalized disobedience on the entire course Elimination
B. Falls -
(1) First fall of competitor Elimination
(2) First fall of horse Mandatory retirement
C. Willful Delay - (Beginner Novice, Novice and Training Levels)
(1) Between the last fence and the finish line 20 penalties
ETA link: http://useventing.com/resources/file...k_2011_web.pdf
Read more: Rule regarding a fall on XC
Hope that helps!